Nanoparticles (i.e., particles having 1–20 nm diameter) have been the subject of intense research during the last decade due to their novel electronic, catalytic, and optical properties. As an example, it has been reported that nanostructured molybdenum sulfide (MoS2) is a good catalyst for hydrogensulfurization (HDS). One method for making nanostructured MoS2 is using sonochemical synthesis. See M. M. Mdleleni, T. Hyeon, K. S. Suslick (1998). “Sonochemical Synthesis of Nanostructured MoS2” J. Am. Chem. Soc. 120: 6189–6190. Cadmium selenide (CdSe) is the most studied material, arguably due to its tunable fluorescence in visible region, potential use in industrial and biomedical applications.
Variations in fundamental properties of nanoparticles can be induced simply by changing the size of the crystals while holding their chemical composition constant. Despite their high potential, very few applications for nanocrystals have been developed, in large part due to difficulty and cost associated with producing uniform nanosize particles in sufficient quantities for exploring new practical applications and processing techniques.
Semiconductor nanoparticles, containing hundreds to a few tens of thousands of atoms show strong size-dependence of their physico-chemical properties. See e.g., Alivisatos, A. P. (1996). “Perspectives on the physical chemistry of semiconductor nanocrystals.” J. Phys. Chem. 100(31): 13226–13239; Eychmuller, A. (2000). “Structure and photophysics of semiconductor nanocrystals.” J. Phys. Chem. B 104(28): 6514–6528; C. B. Murray, C. R. Kagan, M. G. Bawendi (2000). “Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies.” Ann. Rev. Mater. Sci. 30: 545–610; M. Green, P. O'Brien (1999). “Recent advances in the preparation of semiconductors as isolated nanometric particles: new routes to quantum dots.” Chem. Commun.: 2235–2241; T. Trindadae, P. O'Brien, N. L. Pickett (2001). “Nanocrystalline semiconductors: synthesis, properties and perspectives.” Chem. Mater. 13: 3843–3858; K. Grieve, P. Mulvaney, F. Grieser (2000). “Synthesis and electronic properties of semiconductor nanoparticles/quantum dots.” Current Opinion Coll. Interface Sci. 5: 168–172. One particularly evident example of such quantum size effects is the blue-shift of absorbance and fluorescence emission with decreasing size of semiconductor nanoparticles. Potential applications of semiconductor nanoparticles include light emitting diodes (see V. L. Colvin, M. C. Schlamp, A. P. Alivisatos (1994). “Light-emitting diodes made from cadmium selenide nanocrystals and a semiconductor polymer.” Nature (London) 370: 354–357), biological fluorescent labels (see M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A. P. Alivisatos (1998). “Semiconductor nanocrystals as fluorescent biological labels.” Science (Washington D.C.) 281: 2013–2016; and W. C. W. Chan, S. Nie (1998). “Quantum dots bioconjugates for ultrasensitive nonisotopic detection.” Science (Washington D.C.) 281: 2016–2018), solar cells (see W. U. Huynh, J. J. Dittmer, A. P. Alivisatos (2002). “Hybrid Nanorod-Polymer Solar Cells.” Science (Washington D.C.) 295: 2425–2427; and W. U. Huynh, J. J. Dittmer, W. C. Libby, G. L. Whiting, A. P. Alivisatos (2003). “Controlling the morphology of nanocrystal-polymer composites for solar cells.” Adv. Funct. Mater. 13: 73–79), lasers (see V. I. Klimov, A. A. Mikhilovsky, S. Xu, A. Malko, J. A. Hollingsworth, D. W. McBranch, C. A. Leatherdale, H-J. Eisler, M. G. Bawendi (2000). “Optical gain and stimulated emission in nanocrystal quantum dots.” Science (Washington D.C.) 290: 314–317; and H-J. Eisler, V. C. Sundar, M. G. Bawendi, M. Walsh, H. I. Smith, V. I. Klimov (2002). “Color-selective semiconductor nanocrystal laser.” Appl. Phys. Lett. 80: 4614–4616), and catalysts (see T. R Thurston, J. P. Wicoxon (1999). “Phooxidation of organic chemicals catalyzed by nanoscale MoS2.” J. Phys. Chem. B. 103: 11–17).
Various techniques have been used for the synthesis of semiconductor nanocyrstals, for example: arrested precipitation in solutions, synthesis in structured media, high temperature pyrolysis, sonochemical, and radiolytic methods. See, Alivisatos, A. P. (1996). “Perspectives on the physical chemistry of semiconductor nanocrystals.” J. Phys. Chem. 100(31): 13226–13239; Eychmuller, A. (2000). “Structure and photophysics of semiconductor nanocrystals.” J. Phys. Chem. B 104(28): 6514–6528; C. B. Murray, C. R. Kagan, M. G. Bawendi (2000). “Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies.” Ann. Rev. Mater. Sci. 30: 545–610; M. Green, P. O'Brien (1999). “Recent advances in the preparation of semiconductors as isolated nanometric particles: new routes to quantum dots.” Chem. Commun.: 2235–2241; T. Trindadae, P. O'Brien, N. L. Pickett (2001). “Nanocrystalline semiconductors: synthesis, properties and perspectives.” Chem. Mater. 13: 3843–3858; K. Grieve, P. Mulvaney, F. Grieser (2000). “Synthesis and electronic properties of semiconductor nanoparticles/quantum dots.” Current Opinion Coll. Interface Sci. 5: 168–172. However, each of these approaches have significant limitations, the most important one being difficulty of scale-up.
Bawendi et al. in 1993 described a method (hereinafter “the Bawendi method”) for the production of chalcogenide nanoparticles. See C. B. Murray, D. J. Norris, M. G. Bawendi (1993). “Synthesis and characterization of nearly monodisperse CdE (E=S, Se, Te) semiconductor nanocrystallites.” J. Am. Chem. Soc. 115: 88706–8715. The Bawendi method involves high temperature decomposition of organometallic reagents in hot, coordinating solvents. In accordance with the Bawendi method, solutions of dimethylcadmium (Cd(CH3)2) and tri-n-octylphosphineselenide (TOPSe) are injected into hot tri-n-octylphosphineoxide (TOPO) at temperatures from ˜120 to 300° C. The size distribution of particles can be controlled mainly by the temperature of reaction mixture and the length of the reaction time, with larger particles obtained at higher temperature and longer time of reaction.
The Bawendi method is most popular among conventional methods as it allows for the production near monodisperse particles with good luminescent properties. However, one of the limitations of the Bawendi method is the use of hazardous compounds like Cd(CH3)2 especially at high temperatures. It should be recognized that reagents used in the Bawendi method are relatively expensive. For example, around year 2003, 100 ml of tri-n-octylphosphine technical grade (TOPO), 90% purity, from Aldrich Chemical Co., Milwaukee, Wis., costs about $72.50. Around year 2003, the price for 25 g of dimethylcadmium (Strem Chemicals, Inc. of Newburyport, Mass.) was $540.00. Another disadvantage is that the Bawendi method is complicated, not easily reproducible, and difficult to scale up. Peng et. al. have shown recently that the Bawendi method can be modified by changing precursors and the solvents in which the reaction occurs. See L. Qu, Z. A. Peng, X. Peng (2001). “Alternative routes toward high quality CdSe nanocrystals.” Nano Letters 1: 333–337; and Z. A. Peng, X. Peng (2001). “Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.” J. Am. Chem. Soc. 123: 183–184. Cadmium oxide, carbonate or acetate have been used in place of dimethylcadmium. Non-coordinating solvents (dodecene) instead of TOPO can be used with some success. See W. W. Yu, X. Peng (2002). “Formation of high-quality CdS and other II-VI semiconductor nanocrystals in noncoordinating solvents: tunable reactivity of monomers.” Angew. Chem. Int. Ed. 41: 2368–2371. Using different solvents and surfactants, Peng et al. were able to synthesize nanoparticles of different size and morphology. See Z. A. Peng, X. Peng (2001). “Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.” J. Am. Chem. Soc. 123: 183–184; and Z. A. Peng, X. Peng (2002). “Nearly monodisperse and shape-controlled CdSe nanocrystals via alternative routes: nucleation and growth.” J. Am. Chem. Soc. 124: 3343–3353.
Nonetheless, the use of expensive solvents in high temperature reaction mixtures into which aggressive chemicals must be very quickly and reproducibly injected makes such procedures difficult to scale up. In addition, in order to produce nanoparticles of a desired size, the reaction must be stopped by rapid cooling. All of these problems restrict the use of conventional methods on a larger scale necessary for industrial application. The development of new synthetic methods that are able to produce well-defined materials of nanometer size (especially in a continuous flow process) remains a serious challenge.